[Paper Review] Novel electronic behavior driving NdNiO3 metal-insulator transition

M. H. Upton, Yongseong Choi+6 more|arXiv (Cornell University)|Dec 1, 2014

Magnetic and transport properties of perovskites and related materials1 references3 citations

TL;DR

This study reveals that the metal-insulator transition (MIT) in tensile-strained NdNiO3 is driven by a redistribution of electronic density from Ni 3d to Nd 5d orbitals, rather than by charge disproportionation or symmetry changes. Resonant inelastic x-ray scattering and x-ray absorption spectroscopy show decreased Ni $d_{x^2-y^2}$ occupancy and increased Nd charge occupancy, indicating electron transfer that localizes Ni 3d electrons and opens an insulating gap via hybridization with O 2p states.

ABSTRACT

We present evidence that the metal-insulator transition (MIT) in a tensile strained NdNiO3 (NNO) film is facilitated by a redistribution of electronic density and neither requires Ni charge disproportionation nor symmetry change [1, 2]. Given epitaxial tensile strain in thin NNO films induces preferential occupancy of the $e_g$ $d_{x^2-y^2}$ orbital ($s_{3z^2-r^2}$) we propose the larger transfer integral of this orbital state with the O 2p mediates a redistribution of electronic density from the Ni atom. A decrease in Ni $d_{x^2-y^2}$ orbital occupation is directly observed by resonant inelastic x-ray scattering below the MIT temperature. Furthermore, an increase in Nd charge occupancy is measured by x-ray absorption at the Nd L3 edge. Both spin-orbit coupling and crystal field effects combine to break the degeneracy of the Nd 5d states shifting the energy of the Nd $e_g$ $d_{x^2-y^2}$ orbital towards the Fermi level allowing the A site to become an active acceptor during the MI transition. This work identifies the relocation of electrons from the Ni 3d to the Nd 5d orbitals across the MIT. We propose the insulating gap opens between the Ni 3d and O 2p resulting from Ni 3d electron localization mediated by charge loss. The transition seems neither purely Mott-Hubbard nor simple charge transfer.

Motivation & Objective

• To understand the electronic mechanism driving the metal-insulator transition (MIT) in epitaxial tensile-strained NdNiO3 thin films.
• To determine whether the MIT is driven by charge disproportionation, symmetry breaking, or alternative electronic reorganization.
• To investigate the role of orbital occupancy and charge transfer between Ni and Nd cations across the MIT.
• To clarify the contribution of spin-orbit coupling and crystal field effects in enabling Nd 5d orbital participation in the MIT.
• To identify the nature of the insulating gap formation—whether Mott-Hubbard, charge-transfer, or a hybrid mechanism.

Proposed method

• Epitaxial tensile-strained NdNiO3 thin films were grown on substrates to induce lattice strain.
• Resonant inelastic x-ray scattering (RIXS) was used to probe Ni 3d orbital occupancy changes across the MIT.
• X-ray absorption spectroscopy (XAS) at the Nd L3 edge measured changes in Nd charge occupancy.
• Theoretical analysis considered the role of spin-orbit coupling and crystal field splitting in lifting degeneracy of Nd 5d states.
• Orbital hybridization between Ni 3d, O 2p, and Nd 5d states was evaluated to explain electronic redistribution.
• The MIT was analyzed as a function of temperature to correlate electronic changes with insulating gap formation.

Experimental results

Research questions

• RQ1 Does the MIT in strained NdNiO3 involve Ni charge disproportionation or symmetry breaking?
• RQ2 What is the role of orbital occupancy redistribution in driving the MIT?
• RQ3 How do spin-orbit coupling and crystal field effects enable Nd 5d states to participate in the MIT?
• RQ4 Is the insulating gap formed via Mott-Hubbard or charge-transfer mechanisms?
• RQ5 What is the nature of electron transfer between Ni and Nd cations during the MIT?

Key findings

• A decrease in Ni $d_{x^2-y^2}$ orbital occupation was directly observed below the MIT temperature using resonant inelastic x-ray scattering.
• An increase in Nd charge occupancy was measured at the Nd L3 edge, indicating electron transfer from Ni to Nd.
• The Nd 5d $e_g$ orbital is shifted toward the Fermi level due to spin-orbit coupling and crystal field effects, enabling it to act as an acceptor.
• The insulating gap forms between Ni 3d and O 2p bands due to localization of Ni 3d electrons following charge loss.
• The MIT is neither purely Mott-Hubbard nor simple charge transfer, but driven by a novel electronic redistribution involving Nd 5d participation.
• The transition is mediated by enhanced hybridization between the Ni $d_{x^2-y^2}$ and O 2p orbitals due to increased transfer integrals in the strained state.